FIG. 7 shows a circuit configuration of a conventional three-phase voltage type PWM inverter system composed of a three-phase bridge circuit.
In the figure, reference numeral 11 denotes a DC power supply. Reference character P denotes a positive electrode bus of the DC power supply 11. Reference character N denotes a negative electrode bus of the DC power supply 11. Reference numeral 12 denotes an inverter for converting the DC power of the DC power supply 11 into AC power of a desired frequency and voltage. Reference characters T1-T6 denote switching devices. Reference characters D1-D6 denote diodes in antiparallel connection with the switching devices T1-T6, respectively. Reference numerals 21-23 and 24-26 respectively denote first and second drive power supplies, which are insulated each other so as to drive the switching devices T1-T6 independently. Reference numerals 31-36 denote drive circuits for driving the respective switching devices T1-T6. Reference character Vp denotes a power supply grounded on the positive electrode bus P of the DC power supply 11. Reference characters U, V, and W denote output terminals. Reference numeral 50 denotes a motor driven by the inverter system.
The inverter 12 is constituted by a three-phase bridge circuit which is provided between the positive and negative electrodes of the DC power supply 11 and which is composed of three sets of upper arm circuits (an upper arm circuit formed by the switching device T1 and the diode D1 in antiparallel connection, an upper arm circuit formed by the switching device T2 and the diode D2 in antiparallel connection, and an upper arm circuit formed by the switching device T3 and the diode D3 in antiparallel connection) and lower arm circuits (a lower arm circuit formed by the switching device T4 and the diode D4 in antiparallel connection, a lower arm circuit formed by the switching device T5 and the diode D5 in antiparallel connection, and a lower arm circuit formed by the switching device T6 and the diode D6 in antiparallel connection), drive power supplies 21 through 26, and drive circuits 31 through 36.
FIG. 8 shows a circuit configuration of a conventional three-phase voltage type PWM inverter system which has an inrush current limiting circuit using a thyristor. In the figure, reference numerals and reference characters 12, 21 through 26, 31 through 36, 50, T1 through T6, D1 through D6, P, N, U, V, and W denote the same components as those shown in FIG. 7; hence, the description thereof will be omitted.
Reference characters R, S, and T denote AC power supplies. Reference numeral 13 denotes a power rectifier composed of a diode bridge that rectifies the AC power supplies R, S, and T so as to convert them into DC power. Reference numeral 14 denotes a smoothing capacitor which is connected between buses P and N of the DC power supply and holds a DC voltage, which is an output of the power rectifier 13, at a constant level to ensure smooth control of the inverter 12. Reference character R5 denotes an inrush current limiting resistor for restricting the inrush current into the smoothing capacitor 14. Reference numeral 15 denotes a thyristor for short-circuiting the resistor R5. Reference character R6 denotes a resistor for limiting the gate current of the thyristor 15. Reference numeral 16 denotes a photocoupler for firing the thyristor 15. Reference numeral 17 denotes a control circuit for controlling a firing signal of the photocoupler 16. Reference character Vp denotes a power supply for controlling the gate of the thyristor 15 and or firing the photocoupler 16.
The power supply Vp, the resistor R5, the thyristor 15, he resistor R6, the photocoupler 16, and the control circuit 17 collectively make up the inrush current limiting circuit.
The operation of the inrush current limiting circuit will now be described.
When the AC power supplies R, S, and T are turned ON, charging current toward the smoothing capacitor 14 flows into the power rectifier 13. The smoothing capacitor 14 generally has a large capacitance, so that the charging current reaches to an undue inrush current level.
For this reason, the resistor R5 is usually inserted in a circuit of the smoothing capacitor 14 so as to restrict the inrush current which flows into the smoothing capacitor 14 when the AC power supplies R, S, and T are turned ON, thereby protecting the power rectifier 13.
The smoothing capacitor 14 is charged after a lapse of certain time, then the thyristor 15 is fired to short-circuit the both ends of the resistor R5 so as to start the operation. The thyristor 15 is fired (turned ON/OFF) by the resistor R6, the photocoupler 16, and the control circuit 17. Current is supplied to the gates of the photocoupler 16 and the thyristor 15 from the power supply Vp such as a switching regulator grounded on the positive electrode bus P of the DC power supply.
FIG. 9 shows the configuration of a current detection circuit for detecting the current of the positive electrode bus of the conventional three-phase voltage type PWM inverter.
In the figure, reference numerals and reference characters 11, 12, 21 through 26, 31 through 36, 50, P, N, U, V, and W denote the same components as those shown in FIG. 7, and therefore, the description thereof will be omitted.
Reference character Po denotes a positive electrode bus of the DC power supply 11. Reference character R7 denotes a resistor for detecting current that is connected in series between the positive electrode buses P and Po of the DC power supply 11. Reference numeral 18 denotes an insulating amplifier for insulating the voltage of the resistor R7 from the positive electrode bus of the DC power supply 11. Reference numeral 19 denotes a detection circuit for detecting current. Reference character Vp denotes a power supply for driving the insulating amplifier 18 and it is grounded on the positive electrode bus Po.
The input section of the insulating amplifier 18 is connected to both ends of the resistor R7, while the output section thereof is connected to the detection circuit 19. The positive electrode of the power supply Vp is connected to a power input section A of the insulating amplifier 18.
The operation of the current detection circuit will now be described.
When current flows into the positive electrode of the DC power supply 11, a voltage is generated at both ends of the resistor R7 connected in series between the positive electrode buses P and Po of the DC power supply 11. The voltage generated at both ends of the resistor R7 is supplied to the detection circuit 19 via the insulating amplifier 18 so as to detect the current flowing into the positive electrode bus of the DC power supply 11.
The conventional power converter such as an inverter system or high power factor converter has been configured as discussed above. Hence, if a protecting circuit or the like adopts a power supply grounded on the positive electrode of a DC power supply, then another power supply Vp such as a switching regulator grounded on a positive electrode bus of a DC power supply 11 is required, thereby posing a problem in that the circuit size inevitably becomes larger with resultant higher cost.